1. Field of the Invention
This invention relates to a spring drum for driving a web-like member wound thereon, and more particularly to a spring drum adapted to wind up a shutter blind of a focal plane shutter in a photographic camera. The spring drum is first rotated overcoming the force of a spring provided therein to charge the spring force and then is released to rotate by the charged force of the spring.
2. Description of the Prior Art
The spring drum used in a focal plane shutter for winding up a shutter blind comprises a cylindrical drum member and a coil spring mounted therein one end of which is fixed to the drum member and the other end of which is fixed to a shaft on which the drum is rotatably mounted. In the conventional spring drum, the coil spring is apt to be deformed in a wave form when the spring is charged up. The deformation of the coil spring shortens the life thereof and causes the drum to rotate at irregular speeds. When the drum is rotated by the charged force of the coil spring, the deformed coil spring rubs on the internal surface of the drum and accordingly the rotation of the drum is made unstable or irregular. The irregular rotation of the spring drum is a serious problem in a photographic camera since it degrades the accuracy in control of the shutter speed.
It has been known in the art to apply a lubricant such as molybdenum disulfide or graphite to the internal surface of the drum to reduce the friction between the coil spring and the spring drum as disclosed in Japanese Utility Model Publication No. 981/1975. However, the spring drum with the lubricant only reduces the friction, but does not prevent the coil spring from being deformed. Therefore, the irregularity in the rotation of the drum cannot be completely eliminated.
Thus, it is desired to provide a spring drum which is rotated at a far more accurately uniform speed.